Introduction: Two Nickel-Chromium Alloys, Two Missions
nickel alloy UNS N06600 and nickel-based alloy 690 are often discussed together because they share a nickel-chromium family heritage, yet their chromium levels separate them sharply in performance. nickel alloy UNS N06600 contains 14-17% chromium and is a dependable, general-purpose alloy for high-temperature service, while nickel-based alloy 690 contains 27-31% chromium and delivers markedly better resistance to oxidizing acids, high-temperature water, and stress corrosion cracking (SCC). The practical consequence is that 690 has become the standard replacement for 600 in nuclear steam generators and other extreme environments where 600's resistance is no longer adequate.
Composition and Physical Properties
The table below compares the nominal composition limits of nickel alloy UNS N06600 (UNS N06600) and nickel-based alloy 690 (UNS N06690) plate as covered by ASTM B168.
Element | nickel alloy UNS N06600 (UNS N06600) | nickel-based alloy 690 (UNS N06690)
Nickel (Ni) | 72.0 min | 58.0 min
Chromium (Cr) | 14.0-17.0 | 27.0-31.0
Iron (Fe) | 6.0-10.0 | 7.0-11.0
Carbon (C) | 0.15 max | 0.05 max
Manganese (Mn) | 1.0 max | 0.50 max
Silicon (Si) | 0.50 max | 0.50 max
Copper (Cu) | 0.50 max | 0.50 max
Sulfur (S) | 0.015 max | 0.015 max
Aluminum / Titanium | - | 0.50 max each
The low carbon limit of 690 is deliberate: it minimizes carbide precipitation at grain boundaries and supports resistance to intergranular attack and SCC in service.
nickel-based alloy 690 is slightly less dense than 600 (8.19 versus 8.47 g/cm3) and has a lower melting range, roughly 1343-1377°C versus 1370-1425°C. The two alloys are close in elastic modulus (about 211-214 GPa), thermal expansion (about 13×10⁻⁶/K from 20-100°C), and Poisson's ratio (0.29), and both are essentially non-magnetic in the annealed condition. The practical differences appear in service, not in the basic mechanical framework: 690 combines its corrosion advantages with adequate high-temperature strength, while 600 offers slightly better thermal conductivity (about 14.8 W/(m·K) versus 13.0 W/(m·K) at 20°C) and slightly lower electrical resistivity.
Corrosion Resistance and the Shift from 600 to 690
Chromium is the element that controls corrosion behavior in this alloy family. With roughly twice the chromium of 600, nickel-based alloy 690 forms a more stable passive film and stands up far better to strong oxidizing acids, including nitric acid and mixed acid environments, and to high-temperature water containing chlorides and caustic alkalis. nickel alloy UNS N06600, by contrast, is vulnerable to intergranular attack and stress corrosion cracking in high-temperature aqueous environments, a weakness that proved decisive in nuclear steam generators. Field experience has driven a clear application shift: 690 now replaces 600 (and in many cases 601) in critical harsh environments, becoming the established material for nuclear steam generator tubing and other components where long-term reliability is non-negotiable. For general high-temperature service without extreme aqueous corrosion, 600 remains a cost-effective and proven choice.
Typical Applications
nickel alloy UNS N06600 plate is used for equipment linings, heat exchanger parts, furnace structures, reaction vessels, and components in chemical processing, heat treatment, and aerospace service, where its oxidation resistance and strength from cryogenic to about 1093°C (2000°F) are sufficient. nickel-based alloy 690 plate is specified for nuclear steam generator partitions and linings, spent fuel reprocessing equipment, high-temperature chemical vessels, and strongly alkaline environments, where SCC resistance and long-term stability decide the design. Petrochemical heaters and reformers also select 690 when the process stream couples high temperature with corrosive constituents that would attack lower-chromium alloys.
Selection Guide
Choose nickel alloy UNS N06600 when the service is dominated by high temperature and mild corrosion: furnace hardware, heat treatment fixtures, and general chemical equipment where cost matters and aqueous SCC is not a risk. Choose nickel-based alloy 690 when the environment is oxidizing, aqueous, chloride-bearing, or alkaline at temperature, or when the component carries safety-critical duty in the nuclear industry. Verify the required product form (plate, sheet, tube, bar) against ASTM B168 or the equivalent ASME specification, and confirm that chemistry, heat treatment, and test certificates match the project specification before fabrication.
Frequently Asked Questions
What is the main difference between nickel alloy UNS N06600 and nickel-based alloy 690? Chromium content. nickel alloy UNS N06600 contains 14-17% chromium and is a general-purpose high-temperature alloy; nickel-based alloy 690 contains 27-31% chromium and provides far better resistance to oxidizing acids, high-temperature water, and stress corrosion cracking.
Why has nickel-based alloy 690 replaced nickel alloy UNS N06600 in nuclear steam generators? nickel alloy UNS N06600 experienced intergranular attack and stress corrosion cracking in high-temperature water, leading to field failures. nickel-based alloy 690's high chromium content resists these mechanisms, so it became the standard steam generator material for new plants and replacement bundles.
Are nickel alloy UNS N06600 and 690 weldable? Both alloys are weldable with matching nickel-based filler metals using conventional GTAW and GMAW processes. Clean surfaces, controlled heat input, and proper joint preparation are required, and 690 is commonly welded with a higher-chromium filler to preserve corrosion resistance.
What standards cover nickel alloy UNS N06600 and 690 plate? ASTM B168 covers sheet, strip, and plate for both alloys; ASTM B166 covers rod and bar for 600-type alloys. The UNS designations are N06600 for nickel alloy UNS N06600 and N06690 for nickel-based alloy 690, with corresponding ASME specifications for pressure-boundary service.
Can nickel alloy UNS N06600 be used at cryogenic temperatures? Yes. nickel alloy UNS N06600 retains good toughness and strength down to cryogenic temperatures and maintains its oxidation resistance up to about 1093°C (2000°F), giving it one of the widest useful temperature ranges in the nickel alloy family.
Is nickel-based alloy 690 resistant to nitric acid? Yes. The high chromium content of 690 gives excellent resistance to oxidizing acids such as nitric acid, which is why it is used in spent fuel reprocessing and other applications handling hot, concentrated nitric acid streams.
